to be honest the motor that would require the least amount of work to get in would be a SBC if the Luv you have is a 2wd, Summit has mounting kits to mount them in the Luv, and Hooker sells headers for the swap, they also have a trans mount too.
The only 4-bangers you can but in there with out a crap load of fab work would be the 4ZE1 4-bangers made by Isuzu. The 4ZE1 is a 2.6L MPI putting out about 120hp/150lbs tq stock, but there are a number of upgrades out there that you can add on to get close to 200hp, this motor in theory should bolt right in to the stock motor mounts, you'd be good to grab the 5-speed that came behind that engine, to get that to work you'd have to switch the clutch over to a hydro setup, and have a new driveshaft made. and then there is the job of rewiring the truck to make the MPI work. and having to address the fuel issues, and cooling issues.
the other choice would be a G200 out of a later Isuzu Pup, it's a drop in, in the purest sense. not much of an upgrade though, it only makes about 90hp and 100+-tq, all it is really is a bored out G180. Now what you could do is grab a G200 block and then put a G180 head on it, thus uping you compression ratio, and giving you a bit more power, add a Weber carb on top, and a Pacesetter header, and you'd be in the 120-130hp range, that right there would get the truck out of it's own way, just don't expect to dust any imports off the line. \
If you want to have the most bang for the buck, go with a SBC. They're cheap to build, and cheap to put in, build it mild and you can have an easy 250-300hp, and 300+tq, build it a little hotter and you will be dusting imports left and right without even trying.
Sorry for the long post, but you did ask for info
